Capacitance type manometers are presently the most accurate pressure gages available for measuring very low pressures, i.e., degree of vacuum. Due to their great accuracy capacitance manometers have become the primary standard for pressure measurements in vacuum systems of various types. One type of vacuum system in which capacitance manometers are used are plasma etching systems which perform a step in the manufacture of integrated circuits. As is well known in such systems, various materials are removed from silicon wafers in circuit patterns formed in a previous photoresist developing step. In such plasma etching systems control and, therefore, accurate measurement of the vacuum in which the etching must be carried out is critical.
However, capacitance manometers are subject to zero drift i.e., readings which deviate from true pressure. Zero drift may be the result of hysteresis relaxation, temperature variation, radio frequency pick-up and particularly chemical contamination.
Previously, this problem has been corrected by removing the manometer from a system, measuring it against a known reference and recalibrating it. This is done periodically or whenever there is a suspicion that the manometer has been providing inaccurate readings. Such a procedure requires that the vacuum system be out of use for periods of time. In certain systems such as etching systems where throughput of wafers is important from the standpoint of economy and efficiency, such delays are unacceptable.
The present invention is an apparatus which automatically compensates for deviation of a capacitance manometer from true pressure.